1. Field of the Invention
The present invention relates to a frequency multiplexing circuit and more particularly, to a circuit, in an optical video disc recorder, for cancelling various noises produced when a recording signal, obtained by frequency-multiplexing a second signal comprising, for example, a digital sound signal, on the low frequency band of a first signal comprising a frequency-modulated video signal, is limited through a limiter.
2. Description of the Prior Art
In the conventional optical video disc recorder, a second signal comprising a frequency-modulated sound signal or a digital sound signal is frequency-multiplexed on the low frequency band of a first signal obtained by frequency-modulating a video signal to generate a recording signal. The recording signal is further limited through a limiter and then, supplied to optical modulator means. The optical modulator means is responsive to the recording signal as supplied for intermittently irradiating a laser beam onto a rotary disc to form pits on the surface of the rotary disc. Such an optical video disc recorder is disclosed in, for example, Japanese Patent Publication NO. 29562/1978.
However, in such a conventional optical video disc recorder, three kinds of noises are included in the second signal in the recording signal, as described below.
FIG. 1 is a diagram showing a frequency spectrum for explaining the above described frequency multiplexing of the second signal on the low frequency band of the first signal. In FIG. 1, the axis of the abscissa shows the frequency and the axis of the ordinate shows the signal level. In FIG. 1, fc, in the center of the axis of the abscissa, represents a carrier frequency of the first signal.
In general, in the optical video disc recorder, a low-frequency component of the first signal is previously removed by using, for example, a filter and then, the second signal is frequency-multiplexed so that the low-frequency component of the first signal does not affect a frequency band of the second signal. Thus, a frequency band of the recording signal, after frequency multiplexing, is adapted such that a band A of the first signal does not overlap with a band B of the second signal, as shown in FIG. 1.
However, if the low-frequency component of the first signal is previously removed before frequency multiplexing as described above, a sideband of a frequency-modulated video signal of the first signal is included in the second signal after frequency multiplexing to generate harmonic noise.
More specifically, as shown in FIG. 1, the first signal essentially includes first harmonics in the positions of fc.+-.fm (m:modulating frequency) and second harmonics in the positions of fc.+-.2fm, symmetrically with respect to the carrier frequency fc of the first signal in each case. In addition, the first signal also includes third harmonics in the positions of fc.+-.3fm (not shown). More specifically, as obvious from FIG. 1, the video signal of the first signal includes a sideband generated in the position of, for example, fc-2fm in the frequency band B of the second signal. Such a sideband of the low frequency band can be certainly removed in advance by the filter before frequency multiplexing.
However, the sideband of the video signal once removed by the filter is produced again at a level which is approximately half of the original level in the second band B of the recording signal by applying, to a limiter, the recording signal after frequency multiplexing to make the amplitude thereof constant. Such a sideband, as produced, again deteriorates the SN ratio of the second signal in the recording signal.
FIG. 2 is a waveform diagram for explaining the process of frequency-multiplexing a first signal and a second signal to generate a recording signal, where FIG. 2(a) shows the first signal (a sinusoidal wave) obtained by frequency-modulating a video signal, FIG. 2(b) shows the second signal comprising a frequency-modulated sound signal, FIG. 2(c) shows a signal obtained by frequency-multiplexing the first signal and the second signal, and FIG. 2(d) shows a recording signal obtained by limiting the amplitude of the signal shown in FIG. 2(c) by a limiter. Pits are formed on the surface of a rotary disc corresponding to this recording signal (d) as a two-level signal.
Thus, as obvious from FIGS. 2(c) and 2(d), a pit forming pitch, i.e., the distance between the centers of the adjacent pits is defined by the first signal of a sinusoidal wave, and a duty cycle of each pit is defined by the second signal. More specifically, if the first signal linearly changes as a triangular wave, it is considered that the duty cycle of the pit changes in direct proportion to the level of the second signal (b). However, as shown in FIG. 2(a), if the first signal is a sinusoidal wave, a portion which linearly changes is only a part of the entire waveform, so that the duty cycle of the pit does not change in direct proportion to the level of the second signal (b). Consequently, linear distortion noise is produced in the recording signal.
FIG. 3 is a graph for explaining the principle of generating linear distortion noise, where the axis of the abscissa shows the amount of change in the duty cycle and the axis of the ordinate shows the level of the second signal. In FIG. 3, a straight line X, represented by a broken line, shows an ideal characteristic of the amount of change in the duty cycle when the first signal linearly changes in all portions. On the other hand, a curved line Y, represented by a solid line, shows a characteristic of the amount of change in the duty cycle when the first signal is a sinusoidal wave as shown in FIG. 2(a). For example, in the ideal characteristic represented by the straight line X, the amount of change in the duty cycle must be "B" when the level of the second signal is "A". However, in the actual characteristic represented by the curved line Y, the corresponding amount is "C". As a a result, modulation distortion of (C-B) occurs. More specifically, as obvious from FIG. 3, the linear modulation distortion increases as the level of the second signal increases.
Then, asymmetrical noise caused by asymmetry of an output of the limiter may be included in the recording signal. More specifically, when a slice level of the limiter, for limiting the amplitude of the recording signal to a constant level, is not correctly set, the amplitude of a frequency-modulated signal applied to the limiter is asymmetrically limited. As a result, a DC component is multiplexed on the output of the liminator.
FIG. 4 is a diagram showing a frequency spectrum for explaining the principle of generating such an asymmetrical noise, where the axis of the abscissa represents the frequency and the axis of the ordinate represents the signal level. If the DC component is multiplexed on the output of the limiter as described above, it is known that a signal component having a frequency of fm is produced on the low frequency band of the first signal. More specifically, as obvious from FIG. 4, the signal component having a frequency of fm becomes asymmetrical noise included in the frequency band B of the second signal, which deteriorates the SN ratio of the second signal in the recording signal.
When the recording signal is recorded on the rotary disc while including harmonic noise, linear distortion noise and asymmetrical noise as described above, the SN ratio of the second signal, in a signal reproduced from the rotary disc, is also deteriorated.
As described in the foregoing, in the conventional optical video disc recorder, even if a low-frequency component of the first signal is previously removed before frequency multiplexing, three kinds of noises are included in the recording signal, so that the SN ratio of the recording signal and the reproduced signal is deteriorated.